Exposure compensating device for radiographic apparatus

ABSTRACT

A device in radiographic apparatuses for compensating the variations in thickness, density and absorption properties in different parts of an object being radiographed so as to produce a more uniform average exposure of the radiographic recording medium and thereby a more uniform image contrast in all parts of the radiograph of the object. The device comprises a compensating filter device inserted in the radiation path between the radiation source and the object and including radiation absorbing means, which has a variable shape or form such that its absorption values within different portions of the radiation beam can be varied substantially independently of each other. The shape of this radiation absorbing means is varied by automatically operating control means in response to output signals from radiation detecting means disposed on the opposite side of the object so as to sense the average intensity values in different sections of the radiation beam leaving the object to be radiographed.

United States Patent [191 Edholm et al.

[ Aug. 28, 1973 EXPOSURE COMPENSATING DEVICE FOR RADIOGRAPHIC APPARATUS[75] Inventors: Paul Edholm, Linkoping; Nils Bertil Jacobson, Solna,both of Sweden [73] Assignee: Medinova AB, Solna, Sweden [22] Filed:Nov. 24, 1971 [21] Appl. No.: 201,676

[30] Foreign Application Priority Data Nov. 30, 1970 Sweden 16209/70 52Us. Cl ..zs0/322, 250/358,250/482 [51] Int. Cl H05g 3/00 [58] Field ofSearch 250/86, 65 R [56] References Cited FOREIGN PATENTS ORAPPLICATIONS 1,145,277 3/1963 Germany 250/86 816,845 5/1937 France250/86 Primary Examiner-James W. Lawrence Assistant Examiner- C. E.Church AttorneyFrederick E. Hane et al.

[ 57] ABSTRACT A device in radiographic apparatuses for compensating thevariations in thickness, density and absorption properties in differentparts of an object being radiographed so as to produce a more uniformaverage exposure of the radiographic recording medium and thereby a moreuniform image contrast in all parts of the radiograph of the object. Thedevice comprises a compensating filter device inserted in the radiationpath between the radiation source and the object and including radiationabsorbing means, which has a variable shape or form such that itsabsorption values within different portions of the radiation beam can bevaried substantially independently of each other. The shape of thisradiation absorbing means is varied by automatically operating controlmeans in response to output signals from radiation detecting meansdisposed on the opposite side of the object so as to sense the averageintensity values in different sections of the radiation beam leaving theobject to be radiographed.

31 Claims, 12 Drawing Figures PATlsml-tmuses 1913 3 755-672 SHEET U BF 8PATENTEUAUB28'975 3.755.672

SHEET 7 [IF 8 PATENTEDmszamn SHEET 8 BF 8 Fig. 11

EXPOSURE COMPENSATING DEVICE FOR RADIOGRAPHIC APPARATUS The presentinvention relates to radiographic apparatuses and more particularly to adevice in radiographic apparatuses for equalizing the average exposureor average radiation intensity in the image plane of the apparatus sothat the average exposure is made substantially uniform over the entirearea of the image recording medium being used.

As well known in the art, a radiographic apparatus comprises as itsfundamental components a radiation source, normally an X-ray tube, anobject plane in which the object to be radiographed is positioned, andan image plane on the opposite side of the object plane relative to theradiation source, in which image plane an image recording medium ordevice is disposed. This image recording medium may for instance be afilm sensitive to the radiation, a fluorescent display screen or anelectronic image intensifier. An important problem in radiographicapparatuses is caused by the fact that the average intensity indifferent portions of the radiation beam leaving the object beingradiographed and thus the average exposure of the correspondingdifferent portions of the image recording medium displays often verylarge variations caused by differences in thickness, density andabsorption properties in different portions of the object. Due to thisit is often impossible to obtain an exposure within the prescribedexposure range of the image recording medium, the so called exposurelatitude, over the entire area of the image recording medium. Thus, someparts of the radiograph may be over-exposed, whereas other parts may beunder-exposed, wherefore in these parts the image contrast isinsufiicient to give the desired and necessary information regarding thecorresponding portions of the object being radiographed. The most widelyused method of overcoming this problem is to make two or moreradiographs of the object with different radiation intensities and/ordifferent exposure times for the different radiographs. This method has,however, i.e. the disadvantages that the total time necessary for theradiographic examination is prolonged, that the film costs are increasedand that the object, eg a human patient, is exposed to a larger totalradiation dose. Further, when making a radiograph of the thickestportions of the object, in which case a high radiation intensity isused, the thinner portions of the object, e.g. the patient, are exposedto an unnecessarily large radiation dose. This unnecessarily highradiation intensity in the thinner portions of the object produces alsoa high level of scattered secondary radiation, which causes a diffusedexposure or background fogging of the image recording medium, which alsoresults in a reduced image contrast.

To overcome these disadvantages it has been suggested in the art toequalize the average radiation intensity in different portions of theimage plane so that the average radiation intensity and thus the averageexposure is made substantially uniform within different portions of theimage plane and the image recording medium, respectively. For thisobject twodifferent methods have been suggested. In the one methodradiation absorbing bodies, generally of metal, are positioned in theradiation path between the radiation source and the object plane; theshape, the thickness in the direction of radiation and the position inthe radiation beam of Pat. specification No. l 535 359, the German Pat.specification No. 1 079 448 and the Swiss Pat. specification Nos. 243731 and 254 461.

The other method suggested in the prior art consists therein that adiaphragm, oscillating or rotating in a plane perpendicular to thedirection of radiation, is disposed in the radiation path generallybetween the radiation source and the object plane. The shape, thepositon relative to the radiation beam and the oscillating or rotatingmovement of this disphragm are selected in such a manner that the totalexposure times for the different portions of the object becomesubstantially proportional to the absorption values of said differentportions of the object. Devices of this type are disclosed in e.g. theSwiss Pat. specification No. 154 209 and the German Pat. specificationNos. 1 023 315, l 193 796 and l 017 024.

Prior art devices of the two types discussed above have as a commonserious disadvantage that specifically shaped absorption bodies ordiaphragms, respectively, are necessary for each type of objects to beradiographed, as for instance skull, trunk, extremities, etc. Further,the absorption bodies or diaphragms, respectively, must be positionedmanually relative to the radiation beam on the basis of an estimation ofthe absorption in different portions of the object to be radiographed.This manual operation is time-consuming and requires skilled personneland gives, even in the best cases, only a rough and relativelyunaccurate equalization of the absorption differences in differentportions of the object and chiefly only of absorption differences due todifferences in the dimension of the object in the direction ofradiation, i.e., in the thickness of the object. Therefore, in this wayit has not been possible to compensate for absorption differences causedby differences in the structure of different portions of the object.Further, it has only been possible to compensate for geometrically largeabsorption differences in the object, i.e., absorption differenceshaving an extension within the object substantially corresponding to theextension of the object itself in the direction perpendicular to thedirection of radiation. Geometrically more closely related absorptiondifferences, i.e., with a higher spatial frequency, have not beenpossible to compensate.

The object of the present invention is therefore to provide an improveddevice in radiographic apparatuses for equalizing the average exposurein the different portions of the image plane, said device being of thegeneral type known in the prior art and discussed above, which comprisesa radiation absorbing compensating filter device inserted in theradiation path between the radiation source and the object plane andhaving different absorption values within different portions of a planesubstantially perpendicular to the direction of radiation. However, thedevice according to the invention is automatic in its operation andprovides a more accurate and a finer equalization of the averageexposure in the image plane.

The device according to the invention comprises a compensating filterdevice including radiation absorbing means having a variable shape orform so that its absorption values within different portions of theradiation beam from the radiation source can be varied and selectedindependently; control means for determining the absorption values ofsaid compensating filter device within different portions of theradiation beam by variation of the shape of said radiation absorptionmeans in response to control signals supplied to said control means; andradiation detecting means located beyond the object plane as seen fromthe radiation source for sensing the average intensity values withindifferent portions of the radiation beam and generating output signalsrepresenting said average intensity values; the control signals for saidcontrol means detennining the absorption values of said compensatingfilter device within different portions of the .radiation beam beingderived from the output signals of said radiation detecting means.

As in the device according to the invention the compensating filterdevice includes radiation absorbing means with a variable shape so thatits absorption values within different portions of the radiation beamcan be varied substantially independently, and the control means varyingthe shape of said radiation absorbing means and thus determining saidabsorption values of the compensating device are responsive to theoutput signals of radiation detecting means measuring the averageintensity values of different portions of the radiation beam leaving theobject, an automatic equalization of the average radiation intensity inthe image plane is provided with an accuracy as high as permitted by thedesign of the radiation absorbing means being used. Consequently, theaverage exposure is equalized completely automatically on the basis of aquantative measuring of the radiation intensity values within differentportions of the radiation beam leaving the object. The limit for theequalization of the average exposure is determined substantially only bythe design of the compensating filter device. With a filter deviceaccording to the invention consisting fundamentally of a layer of aformable radiation absorbing substance having a thickness in thedirection of the radiation that can be varied within different portionsor sections of the layer, it is possible to achieve a very accurate andcomplete compensation for different absorption values in differentportions of the object being radiographed. As compared with prior artdevices for the same object the device according to the invention has asadditional advantages that it does not require any manual adjustments,which saves time and calls for a less skill of the personnel, thatdifferently shaped absorption bodies are no longer necessary fordifferent types of objects being radiographed, that a correct averageexposure of the image recording medium being used can be obtained withinall sections of the image so that structure details in the object can bediscerned in all sections of the image in spite of the restrictedworking range of the image recording medium, and that the totalradiation dose to which the object, the patient, is exposed will belower than at a less complete equalization of the expo sure. In thefollowing the invention will be further described with reference to theaccompanying drawings, which show by way of example a number ofembodiments of a device according to the invention. In the drawings FIG.1 illustrates schematically the fundamental layout of a device accordingto the invention;

FIG. 2 illustrates schematically a first simple embodiment of a deviceaccording to the invention, in which the radiation absorbing means inthe compensating filter device consists of a number of solid bodies ofradiation absorbing material, which can be moved to varying positionsrelative to each other and the radiation beam;

FIG. 3 shows schematically a somewhat more sophisticated deviceaccording to the invention with a compensating filter device comprisingsolid bodies of radiation absorbing materials, which can be movedrelative to each other and the radiation beam;

FIG. 4 is a schematical side view partially in section of a compensatingfilter device according to the invention, in which the radiationabsorbing means consists of a radiation absorbing liquid enclosed in aflat chamber, which is disposed substantially perpendicular to thedirection of radiation and the thickness of which can be varied withindifferent portions of the radiation beam;

FIG. 5 is a plan view partially in section of the compensating filterdevice illustrated in FIG. 4;

FIG. 6 illustrates schematically the design of the flexible diaphragmforming one wall in the liquid-filled chamber in the compensating filterdevice shown in FIGS. 4 and 5;

FIG. 7 is a section through said diaphragm along the line VII-VII inFIG. 6;

FIG. 8 is a section through said diaphragm along the line VIII-VIII inFIG. 6;

FIG. 9 illustrates schematically an embodiment of a device according tothe invention, in which the compensating filter device includes a layerof a formable or moldable, radiation absorbing material disposed on aflat tray;

FIG. 10 illustrates schematically in cross-section a compensating filterdevice according to the invention, in which the radiation absorbingmeans consists of a layer of a radiation absorbing, intrinsically loosepowder, which is supported on a flat tray and maintained in a stable,moldable or formable shape by an airpressure gradient across the layer;

FIG. 11 illustrates schematically still another embodiment of a deviceaccording to the invention, in which the compensating filter devicecomprises radiation absorbing means consisting of a layer of a radiationabsorbing, intrinsically loose powder, which is deposited with a varyingthickness upon a flat tray and is maintained in a stable unmoving stateby an airpressure gradient produced across the'powder layer on the tray;and

FIG. 12 illustrates schematically a device for depositing the radiationabsorbing powdered material on the tray in the compensating filterdevice illustrated in FIG.

Due to the varying thickness and composition of the object 3 the averageradiation intensity will, as discussed in the foregoing, be differentwithin different portions of the radiation beam leaving the object 3,which can cause unacceptably large variations in the average exposurewithin the different portions of the film 5, resulting in thedisadvantages discussed in the foregoing.

In order to compensate for the absorption differences in differentportions of the object 3 and thus equalize the average exposure of thefilm 5 a device according to the invention is provided. This devicecomprises a compensating filter device 6 inserted in the radiation beambetween the radiation source 1 and the object plane 2, preferablyadjacent the radiation source 1. The compensating filter device 6 isillustrated only very symbolically in FIG. 1, but a number of differentembodiments of such compensating filter device will be described in thefollowing. The fundamental characteristic feature of this compensatingfilter device is that it includes absorption filter means, which can bevaried as to shape or form in such a way that the absorption valueswithin different portions of the radiation beam can be varied orselected substantially independently but without any localdiscontinuities in the absorption,

which could produce corresponding shadow images on the film 5. Further,the compensating filter device 6 is provided with or coupled toelectrically controlled control means 7, by means of which theabsorption values of the compensating filter device 6 within differentportions of the radiation beam can be determined'in response to controlsignals supplied to the control means 7. The device according to theinvention comprises also radiation detecting means 8a 8e disposed beyondthe object plane 2 as seen from the radiation source 1 so as to sense ormeasure the average intensity values in different portions of theradiation beam as affected by the compensating filter device 6 and theobject 3. These radiation detecting means produce output signalsrepresenting said intensity values. In the example illustrated in FIG. 1said radiation detecting means includes six separate radiation detectors8a 8e, which can sense or measure the radiation intensity within sixdifferent portions of the radiation beam; it being assumed that thecontrol means 7 can vary the absorption values of the compensatingfilter device 6 within the same six different portions of the radiationbeam. The output signals from the radiation detectors 8a 8e areconnected to corresponding differential amplifiers 9a 9e, which alsoreceive a common reference or datum signal from a terminal 10. Theoutput signals from the amplifiers 9a 9e are connected as controlsignals to the control means 7 for the compensating filter device 6.

It is appreciated that the device according to the invention constitutesa closed-loop control system, which automatically operates thecompensating filter device 6 to such a setting that the averageintensity in the image plane 4 within the different portions of theradiation beam received by the radiation detectors 8a 8e becomessubstantially constant and assumes a value determined by the amplitudeof the reference signal on the terminal 10.

It is also appreciated that the degree of accuracy and completeness inthe equalization of the average exposure in the image plane 4 is mainlydetermined by the design of the compensating filter device 6 and inparticular by the number of different sections of the radiation beam inwhich the absorption values of the compensating filter device can bevaried or selected independently of each other.

In FIG. 1 the radiation detectors 8a 8e are disposed beyond the imageplane 4 as seen from the radiation source 1'. In this case thecompensating filter device 6 may be adjusted with the film -5 located inits recording position, provided that the film and the film casette aretranslucent to the radiation and the radiation detectors 8a 8e have asufficient sensitivity so that the adjusting of the compensating filterdevice can be carried out with such a low radiation intensity from theradiation source 1 that no image producing exposure of the film 5results. Otherwise, the adjusting of the compensating filter device 6must be carried out without any film 5 in the image plane 4, whereafterthe film is positioned in the image plane and the actual radiographicexposure of the object is carried out. In order to avoid exposing theobject 3, the patient, to an unnecessarily large radiation dose, theadjusting of the compensating filter device 6 is preferably carried outwith a considerable smaller radiation intensity than the intensity usedfor the subsequent radiography of the object on the image recordingmedium being used.

Alternatively, the radiation detectors 8a 8e could of course be disposedbetween the object plane 2 and the image plane 4, in which case thedetectors could either be sufiiciently translucent to the radiation notto produce any shadow images on the image recording medium 5, or theycould be mounted in a manner permitting their removal from the radiationbeam after the adjusting of the compensating filter device 6 but beforethe actual radiographic exposure of the image recording medium.

Further, a device according to the invention may be designed in a mannerpermitting the removal of the filter device 6 from the radiation beam,in which case the filter device may be adjusted when positioned outsidethe radiation beam, whereafter the filter device is moved to a welldefined predetermined position within the radiation beam before theradiographic exposure of the object. In this case, however, there isobviously no closed-loop control system present during the adjustment ofthe compensating filter device, wherefore the different absorptionvalues within the different sections of the filter device must beadjusted by the control 'means 7 in response of the control signals fromthe differential amplifiers 9a 9e on values that are complementary tothe absorption values of the object 3 within its corresponding differentportions.

FIG. 2 shows a simple embodiment of a device according to the invention,in which the pre-adjustable compensating filter device consists of twosolid bodies 11a and 11b of a radiation absorbing material, which can bemoved relative each other and the radiation beam from the radiationsource 1 in a plane substantially perpendicular to the direction ofradiation. As the absorption bodies are wedge-shaped, the degree ofabsorption of the portions of the radiation beam passing through theabsorption bodies can be varied by variation of the positions of theabsorption bodies. The absorption bodies are moved by servomotors 12aand 12b, respectively, which are controlled by the output signals fromthe differential amplifiers 13a and 13b, respectively. These twodifferential amplifiers are driven on the one hand by the output signalsfrom two radiation detectors 14a and 14b, respectively, which areaffected by the portions of the radiation beam passing through theabsorption bodies 1 1a and 11b, and on the other hand by a commonreference signal from a radiation detector 15, which is affected by thecentral portion of the radiation beam, which has an intensity which issubstantially independent of the position of the absorption bodies 11a,11b. Consequently, the two absorption bodies 11a, 11b are automaticallymoved to such positions that all radiation detectors 14a, 14b and 15receive substantially equal radiation intensities, whereby anequalization of the average exposure of the peripheral and centralportions, respectively, of the film is achieved.

FIG. 3 shows an embodiment of the invention adapted for a so calledback-table, that is an apparatus mainly for radiography of the trunk ofa patient 3. Also in this embodiment of the invention the compensatingfilter device consists of a number of solid bodies of radiationabsorbing material, which are movable relative each other and theradiation beam. On each side of the central plane through the patient 3there is provided a series of pivotally interconnected absorption bodies16a, 16b and 16c. For the sake of simplicity the corresponding assemblyof absorption bodies on the opposite side of the central plane throughthe patient 3 is not shown in the drawing. In the illustrated examplethree pivotally interconnected absorption bodies 16a 160 are provided oneach side. These pivotally interconnected absorption bodies may forinstance be of the type described more in detail in the US. Pat.application Ser. No. 1 l3 013. The pivot joints between the absorptionbodies 16a 16c and the free ends of the two outermost absorption bodies16a and 16c are connected to four servomotors 17a, 17b, 17c and 17d,

respectively, in any suitable manner so that the absorption bodies canbe moved in a direction substantially perpendicular to the radiationbeam. Each of these servomotors 17a 17d is driven by the output signalfrom an associated differential amplifier. For the sake of simplicitythe drawing shows only the amplifier 18a for the servomotor 17a. In thesame way as discussed in the foregoing, a numberof radiation detectors19a, 19b, 19c and 19d are located underneath the object plane, where thepatient 3 is positioned, so as to sense the intensity values of theportions of the radiation beam which are affected by the positions ofthe absorption bodies 16a 160. In the illustrated example theseradiation detectors 19a 19d are elongate and extend parallel to thedirection in which the absorption bodies 16a 16c can be moved. Further,an elongate reference detector 20 is provided, which is positioned inthe central plane through the patient 3 and consequently senses theintensity of the central portion of the radiation beam. The outputsignal from this reference detector 20 is used as a reference signal forall servomotors and is consequently connected e.g. to the differentialamplifier 18a for the servomotor 17a. In the same way the output signalsfrom the radiation detectors 19a 19d are used as control signals for theservomotors 17a 17d, respectively, wherefore e.g. the output signal fromthe radiation detector 190 is connected to the amplifier 18a for theservomotor 17a. However, the signals to the servomotor amplifiers, e.g.the amplifier 184, are transferred to the amplifiers through apotentiometer 21 for the reference signal from the reference detector 20and another potentiometer for the output signal from the associatedradiation detector, e.g. the potentiometer 22a for the output signalfrom the detector 194. These potentiometers are operated in response tothe actual position of a primary diaphragm 23 used for restricting theradiation beam from the radiation source 1. This diaphragm consistsfundamentally of four diaphragm plates, which are movable pairwiserelative to each other for determining the size of a rectangulardiaphragm aperture. the potentiometer 21 for the reference signal fromthe reference detector 20 is operated in response to the position of thediaphargm plates determining the size of the diaphragm aperture in thelongitudinal direction of the reference detector 20, whereas thepotentiometers for the output signals from the other radiationdetectors, e.g. the potentiometer 20a for the output signal from thedetector 19a, are operated in response to the position of the diaphragmplates determining the size of the diaphragm aperture in thelongitudinal direction of the radiation detectors 19. It should be notedthat there is provided one potentiomter 22 for each radiation detector19. By means of these potentiometers 21 and 22 a compensation is madefor the screening effect of the primary diaphragm 23 upon the detectors20 and 19. Such a compensation is necessary, as the detectors are notpoint-shaped but elongate. It is appreciated that without such acompensation dependent on the illuminated portion of the radiationdetectors, the positions of the absorption bodies would be changed whenthe setting of the primary diaphragm 23 is changed, which is of courseundesired, as this would give cause to an erroneous positioning of theabsorption bodies.

From the embodiments of the invention shown in FIGS. 2 and 3 anddescribed in the foregoing it is obvious that the degree of accuracy andcompleteness in the equalization of the exposure that may be obtainedwith a device according to the invention with solid absorption bodies isto a large extent dependent on the number of the absorption bodies,their shape, their mutual arrangement and the permissible variations intheir mutual positions. It is also appreciated that it might benecessary to have difierent arrangements of absorption bodies fordifferent types of objects to be radiographed. For a completeequalization of the exposure it may obviously be necessary to have alarge number of mutually movable absorption bodies, which must have such7 a shape and be movable relative each other in such a manner that theydo not produce any discontinuities in the absorption, as suchdiscontinuities would result in corresponding shadow images.Consequently, a compensating filter device consisting of movable solidbodies of radiation absorbing material suffers from certaindisadvantages.

These disadvantges are eliminated to a large extent in a compensatingfilter device of the type illustrated in FIGS. 4 to 8. In this filterdevice the radiation absorbing medium consists of a liquid 24 enclosedin a thin flat chamber 25, which is adapted to be disposed in a planesubstantially perpendicular to the direction of radiation. The radiationabsorbing liquid 24 may for instance be mercury or some other liquidmetal or a solution or stable suspension of a radiation absorbingsubstance, as for instance an aqueous solution of cesium acetate. Theflat chamber 25 has a plane bottom 26 and an upper wall consisting of aresiliently flexible diaphragm 27, for instance of rubber. At itsperiphery the chamber 25 communicates with a container (not illustratedin the drawing) containing the radiation absorbing liquid 24 so that thechamber 25 is always filled with liquid. A number of stiff but flexiblewires 28 are attached to the upper side of the rubber diaphragm 27 indifferent points distributed over the surface of the diaphragm 27 in apredetermined pattern, for instance a triangular grid pattern, asillustrated in F IG. 5. These wires 28 are guided in corresponding ducts29 in a plate-shaped guide member 30 located directly above the liquidchamber 25. The opposite ends of the wires 28 are coupled to separateservomotors 31 disposed about the circumference of the guide member 30.It is appreciated that the wires 28 and the associated guide ducts 29 inthe guide plate 30 cooperate in the same manner as Bowden cables. Thus,by means of the servomotors 31 and the wires 28 coupled thereto thedifferent sections of the flexible diaphragm 27 can either be withdrawnfrom the bottom 26 of the chamber 25, whereby the thickness of theliquid layer 24 is increased, or be pushed towards the bottom 26,whereby the thickness of the liquid layer is reduced. In this way it ispossible to vary the thickness of the liquid layer 24 in the chamber 25and thus the absorption value of the filter device within each sectionof the filter device corresponding to the point of connection of a wire28 to the diaphragm 27. As the diaphragm 27 is resiliently flexible, thethickness of the liquid layer 24 will vary smoothly so that no abruptdifferences in absorption between adjacent portions of the filter devicecan be created.

The flexible diaphragm 27 has preferably a larger rigidity within thoseportions that are enclosed by the junction lines between the connectionpoints of the wires 28 than within the portions along and directlyadjacent said junction lines. This may be achieved with a diaphragmdesigned in the manner illustrated in FIGS. 6 to 8. In this diaphragmthe portions 32 located between the junction lines between theconnection points 33 of the wires 28 are thicker and consequently morerigid and those portions 34 that are located along and directly adjacentthe junction lines between the connection points 33 of the wires.

The liquid chamber 25, the wires 28 and the guide member 30 are made ofmaterials having a low radiation absorption factor and as the totaldimension of these members is substantially uniform and constant overthe entire filter device, these members will not,

give cause to any substantial absorption differences in the radiationbeam.

The servomotors 31 for the wires 28 are of course controlled fromcorresponding radiation detectors located beyond the object plane,substantially in the same way as described in the foregoing inconnection with FIG. 1. Consequently, for each servomotor 31 there mustbe provided a corresponding radiation detector and these radiationdetectors should be arranged in a pattern corresponding to the patternof the connection points of the wires 28 to the flexible diaphragm 27.It is realized that the degree of accuracy and completeness of theexposure equalization can be increased or reduced by increasing orreducing, respectively, the number of wires 28 connected to differentpoints on the flexible diaphragm 27.

This compensating filter device has the disadvantage that it consists ofa large number of components, as the number of radiation detectors andthe number of servo circuits must be equal to the number of differentsections of the filter device, in which the absorption values shall bevariable independently of each other. In this respect a compensatingfilter device of the type illustrated in FIG. 9 should be moreadvantageous.

FIG. 9 shows schematically, in the same way as in the foregoing, aradiographic apparatus including a radiation source 1, the object 3 tobe radiographed and the image recording medium in the form of aradiation sensitive film 5. The pre-adjustable compensating filterdevice includes in this case a flat tray 35 mounted in a planesubstantially perpendicular to the radiation beam and supporting a layerof a formable or moldable compound 36 containing a radiation absorbingsubstance. The moldable compound 36 may for instance consist of a powdermixed with a suitable binding agent so that the particles in the powderadhere to each other, a paste or a jelly. The absorption values of thefilter device within different sections of the radiation beam aredetermined by the thickness of the layer 36 in the correspondingsections of the tray 35, and the thickness of the radiation absorbinglayer can be varied by molding or fonning the upper surface of thelayer. For this purpose the illustrated embodiment comprises a containeror dispenser 37, which contains the moldable radiation absorbingcompound and which can be moved above the tray 35 in the directionindicated by an arrow 38. At its lower rear edge the dispenser 37 isprovided with an elongate slot-shaped discharge opening for theradiation absorbing compound, extending across the tray 35. The loweredge of this discharge opening may preferably be constituted by theplane bottom of the tray 35, whereas the upper edge of the dischargeopening is formed by a resiliently flexible lip or slice 39, forinstance consisting of a rubber band. A number of servomotors 40a 40aare connected to this slice 39 so that it can be moved substantially inthe direction of radiation to different spacings from the bottom 35 ofthe tray at different sections along its length. In that the servomotors40a 40e continuously vary the spacings between the different sections ofthe slice 39 and the bottom of the tray 35, while the dispenser 37 is atthe same time moved in the direction 38 relative the tray 35, it ispossible to produce in the tray 35 a layer of the moldable radiationabsorbing compound 36 with a varying thickness and thus a varyingabsorption.

The servomotors 40a 40e are controlled by signals from correspondingdifferential amplifiers 41a 4le, which receive on the one hand a commondatum or ref erence signal and on the other hand the output signals fromcorresponding elongate radiation detectors 42a 42e, which are locatedunderneath the image plane 5 parallel to each other and to the directionof movement 38 of the dispenser 37.

Further, a diaphragm plate 43 with a slot-shaped aperture 44 is arrangedunderneath the tray 35. The aperture slot 44 is parallel to the slice 39of the dispenser 37 and the diaphragmplate 43 is moved in the samedirection as the dispenser 37 in synchronism therewith in such a mannerthat the portion of the radiation beam passing through the aperture slot44 is identical with the portion of the radiation beam passing throughthe tray 35 and the radiation absorbing layer 36 adjacent the slice 39of the dispenser 37.

This compensating filter device is pre-adjusted in the following manner:When starting the molding or forming of the layer of the radiationabsorbing compound 36 in the tray 35 the dispenser 37 is in a positionfurthest to the right in the drawing. The portion of the radiation beamfrom the radiation source 1 passing through the aperture slot 44illuminates then. the right hand portion of the object 3 to beradiographed and the right hand portions of the radiation detectors 42a422. In response to the control signals from the amplifiers 41a 41a theservomotors 40a 402 will move the slice 39 to such a position that themoldable layer 36 in the tray 35 is given such a thickness that the sumof the absorption in this layer and the absorption in the object 3becomes substantially constant and uniform over the entire portion ofthe radiation beam passing through the aperture slot 44, which meansthat all detectors 42a 42c receive substantially equally large radiationintensities. As the dispenser 37 and the diaphragm 43 are moved to theleft in the drawing, the position of the slice 39 is varied successivelyby the servomotors 40a 40e so that the radiation absorbing layer 36 inthe tray 35 will within all portions of the tray be molded to haveabsorption values which are substantially inversely proportional to theabsorption values of the object 3 within corresponding portions of theradiation beam.

After this pre-adjustment of the compensating filter device thediaphragm 43 is removed from the radiation path so that the whole object3 can be exposed to the radiation beam for the radiography of theobject.

After the radiographic exposure of the object the dispenser 37 isreturned to its initial position, and the dispenser has such a designthat during this return movement the layer of radiation absorbingcompound 36 in the tray 35 is made level.

In the compensating filter device illustrated in FIG. 9 and describedabove it has been assumed that the radiation absorbing compound disposedas a formable layer 36 in the tray 35 has intrinsically such aconsistency, for instance consisting of a paste or a jelly or a powdermixed with a suitable binding agent, that the upper surface of the layercan easily be molded or formed and subsequently maintain its form.However, it has been found that it is also possible to use anintrinsically loose and freely moving powder of a radiation absorbingmaterial, eg. a plastic material containing a radiation absorbingsubstance. The upper surface of a layer of such an intrinsically loosepowder can of course not, without special steps, be molded or formed tothe extent required by the invention with depressions and elevationswith comparatively steep sides. It has been found, however, that it ispossible to transfer such a layer of an intrinsically loose and freelymoving powder into a very stable state, in which the upper surface ofthe layer can be formed or molded with depressions and elevations withvery steep sides, which remain substantially unchanged after the formingor molding process, by creating an air-pressure gradient across thelayer from its upper side to its lower, side. Such. a pressure gradientcan preferablybe produced by providing the tray supporting the powderlayer with a bottom which is air-permeable but impervious to the powderand providing means for generating a reduced airpressure underneath thisforamenous bottom of the tray. The air-permeable bottom of the tray mayfor instance consist of a stretched, finely woven fabric.

FIG. 10 in the drawing illustrates schematically and in section acompensating filter device according to the invention based upon theabove discussed principle. This compensating filter device may be usedfundamentally in the same way as the filter device illustrated in FIG.9.

Thus, the filter device illustrated in FIG. 10 comprises a tray 35,which in the same way as in the filter device according to FIG. 9 isadapted to support a layer 36 of radiation absorbing material, which inthis case consists of an intrinsically loose and freely moving powder.By contrast with the filter device according to FIG. 9, however, thetray 35 in the filter device according to FIG. 10 has a foramenousbottom 45, which is permeable to air but impervious to the powdermaterial 36. Further, a suction chamber 46 is provided underneath theair-permeable bottom 45. This suction chamber 46 is in any convenientmanner only schematically illustrated in the drawing connected to an airpump 47, by means of which a reduced air-pressure can be created withinthe suction chamber 46. As a result of this reduced pressure in thechamber 46 and the resulting air flow through the powder layer 36 andthe airpermeable bottom 45 an air-pressure gradient is created withinthe powder layer 36. Under the effect of this pressure gradient thepowder layer assumes a very stable state so that the upper surface ofthe powder layer can easily be molded or formed with remainingdepressions and elevations with very steep sides.

The forming of the upper surface of the powder layer 36, so as to givethe powder layer the desired varying thickness, is carried out in amanner similar to that in the filter device according to FIG. 9, in thatan elongate resiliently flexible scraper or knife 48, which extendsacross the tray 35, is moved above the tray in the direction indicatedby an arrow 49 at the same time as the scraper 49 is adjusted byservomotors 40a 40e to a desired spacing above the bottom 45 of thetray. As described in the foregoing, this spacing between the scraper 48and the bottom 45 of the tray can be different in different sectionsalong the length of the scraper. During its movement over the tray 35 inthe direction 49 the scraper 48 cuts down in the powder layer 36 andleaves behind it a molded or formed powder layer 36a with a varyingthickness. The excessive powder material at the surface of the originalpowder layer 36 is removed by suction into a container 51 through anelongate suction nozzle 50 located along the upper side of the scraper48. The servomotors 40a 40e controlling the position of the scraper 48are controlled in the same manner as in the filter device illustrated inFIG. 9 by means of signals from the radiation detectors 42a 42c; thescraper 48 being moved over the tray 35 in synchronism with the slotteddiaphragm 43. After the forming or molding of the powder layer 36 theradiographic exposure of the object is carried out in the mannerdescribed in the foregoing. Before a repeated forming of the powderlayer 36 in the tray 35 for radiography of another object, the powdermaterial removed from the tray 35 at the previous forming of the layer36 is replaced so that a powder layer 36 of substantially uniformthickness is recreated, which layer can be formed in the mannerdescribed above.

In the compensating filter device according to the invention illustratedin H6. 10 and described above the radiation absorbing powdered materialis initially arranged in a layer of uniform thickness in the tray 35,whereafter this layer is given the desired varying thickness by removalof a varying portion of the initial layer. However, it is also possibleto deposit the radiation absorbing powdered material upon theair-permeable bottom 45 of the tray 35 already from the beginning in alayer with the desired varying thickness, at the same time as an airpressure gradient is maintained across the deposited powder layer in themanner described in the foregoing so that the powder layer remains in astable state and maintains its varying thickness.

FIG. 11 in the drawing illustrates schematically and by way of example acompensating filter device operating in this manner.

The compensating filter device in FIG. 11 comprises, just as the filterdevice in FIG. 10, a tray 35 having a foramenous bottom 45, which ispermeable to air but impervious to the radiation absorbing powderedmaterial, and a suction chamber 46 arranged underneath the bottom 45 ofthe tray and connected to an air pump 47. For depositing the desiredradiation absorbing powder layer 36 with a varying thickness in the tray35 a device 52 is provided for producing a narrow jet 53 of theradiation absorbing powder directed into the tray 35. This device 52 isassociated with means 54 for moving the powder jet 53 over the entirearea of the tray 35 along a predetermined scanning pattern and also forvarying the intensity of the powder jet, that is the flow rate ofpowdered material in the jet. By moving the powder jet 53 over the tray35, e. g. along a linear scanning pattern, e.g. of the same type as usedin a TV picture tube, and simultaneously modulating the intensity of thepowder jet 53 it is consequently possible to deposit upon the bottom 45of the tray 35 a powder layer having a varying thickness, which ismaintained in a stable unmoving state due to the pressure gradientestablished across the layer.

The radiation detecting means for sensing or measuring the intenstiy ofthe radiation beam leaving the object 3 consists in this case of adevice for electronically scanning the radiation image of the object 3along a predetermined scanning pattern aand generating a video signal,which is proportional at any moment to the intensity of the presentlyscanned portion of the radiation image. As schematically illustrated inFIG. 11 this device for electronically scanning the radiation image ofthe object 3 may as known per se in the prior art include an imageintensifier 55, which converts the radiation image into a correspondingoptical image, and a suitable TV camera tube 56 viewing said opticalimage. As well known in the art the camera tube 56 produces a videosignal, which has an amplitude proportional to the intensity values ofthe scanned points in the radiation image of the object 3. Thisvideosignal from the camera tube 56 is transferred via a signal communication cable 57 to the device 54. Sync signals or other signalsrepresenting the image scanning of the camera tube 56 are alsotransferred to the device 54 via the signal communication cable 57. Inthe device 54 the scanning signals and the video signal from the cameratube 56 are used for controlling on the one hand the scanning motion ofthe powder jet 53 over the tray 35 and on the other hand the varyingintensity of the powder jet in such a manner that the powder jet 53 ismoved over the tray 35 along a scanning pattern corresponding to thescanning pattern of the camera tube 56 and the intensity of the powderjet 53 is varied in correspondence with the varying amplitude of thevideo signal.

FIG. 12 shows schematically and by way of example an embodiment of thedevices 52 and 54 for generating and controlling the powder jet 53 in afilter device of the type illustrated in FIG. 11 and described above.The device 52 for generating the narrow powder jet 53 comprises acylindrical container 58, which is filled with the powdered material andin which a rotateable paddle wheel driven by a suitable motor 60 ismounted. A narrow tube 61 extends into the container 58 through itscircumferential wall so that the inner end of the tube is passed by theblades of the wheel 59. The tips of the blades are provided with notchesfor the passage of the end of the tube 61. By rotation of the paddlewheel 59 the powdered material in the container 58 will be forced outthrough the narrow tube 61 as a narrow restricted powder jet.

The device 54 for controlling the powder jet 53 comprises two pairs ofdeflection plates 62 and 63, respectively, for electrostatic deflectionof the jet 53 in two orthogonal directions. By applying appropriateelectric potentials to the deflection plates 62 and 63 it is possible todeflect the powder jet 53 in a desired direction and by a desired angle,whereby the powder jet 53 can be moved over the tray 35 along a desiredscanning pattern. The necessary deflection voltages for the deflectionplates 62 and 63 are provided by a control unit 64, to which the videosignal from the camera tube 56 is conveyed via the signal communication57. For the intensity modulation of the powder jet 53 an additional pairof electrostatic deflection plates 65 is provided, which can be suppliedfrom the control unit 64 with a deflection voltage with such a largeamplitude that the powder jet 53 is deflected very sharply in thedirection indicated by a dotted arrow 66, whereby the powder jet willnot reach the tray 35 at all but instead be collected in a suitablecontainer not illustrated in the drawing. By pulse modulation of thevoltage supplied to the deflection plates 65 with a pulse rate or a pulsratio varying in response to the video signal from the camera tube 56 itis obviously possible to vary the total amount of powder in the powderjet53 reaching the tray 35 in accordance with the amplitude of the'video signal.

Instead of modulating the intensity of the powder jet 53in the mannerdescribed above it would also be possible to use a powder jet with aconstant flow rate and instead to vary the sweep velocity of the jetover the tray 35 in such a manner that the amount of powder deposited inthe tray 35 within any given portion of the scanning pattern of thepowder jet becomes proportional-to the amplitude of the video signal forsaid given portion of the scanning pattern. It should be noticed that ina filter device of this type the final radiation absorbing powder layer36 in the tray 35 is built-up successively over an interval includingseveral scanning cycles of the camera tube 56 and thus of the powder jet53.

As a device according to the invention makes it possible to achieve avery good equalization of the average exposure of the different portionsof the image recording medium, it becomes possible to use an automaticexposure control system in the radiographic apparatus with a very goodresult. Previous attempts in using automatic exposure control systems inradiographic apparatuses have often given unsatisfactory results, as theautomatic exposure control has frequently been based on the radiationintensity in a section of the image being of minor importance, which hasresulted in an erroneous exposure of the most interesting portions ofthe image, as the average exposure has not been uniform within allportions of the image. In combination with a device according to theinvention, however, which gives a very good equalization of the averageexposure over the entire image, the automatic exposure control systemcan without difficulties be controlled correctly in response to signalsfrom the radiation detectors sensing the radiation intensity in theimage plane.

In some cases it might be advantageous to have a possibility of varyingthe degree of exposure equalization so that only a partial equalizationis obtained. In this way it might be easier to recognize anatomicalstructures from experiences gained from viewing images without anycontrast equalization. Such a variable and partial exposure equalizationcan be obtained with a device according to the invention in that thesevomotors controlling the pre-adjustment of the compensating filterdevice are provided with a negative feed-back from their outputs totheir inputs. In FIG. 2 this is illustrated schematically and by way ofexample for the servomotor 12a, which has its mechanical shaft coupledto the absorption body 110 and also to a suitable signal transducer 45,which produces an electric signal proportional to the angle of rotationof the servomotor 12a and thus to the position of the absorption body11a, this signal being fed back in opposition to the differentialamplifier 13a for the servomotor 12a. By variation of the degree offeed-back it is obviously possible to vary the degree of exposureequalization in the radiographic image.

It is also possible to vary the degree of exposure or contrastequalization by using a radiation with a different energy, that is adifferent voltage on the X-ray tube, for the pre-adjustment of thecompensating filter device before the radiographic exposure of theobject than the radiation used for the subsequent actual radiographicexposure of the object. In this way the degree of contrast equalizationin the radiographic image is changed, as a change in radiation energyresults in unequal absorption changes in the object being radiographedand in the heavy elements constituting the radiation absorbing substancein the compensating filter device. I

The selection of the radiation absorbing substance used in thecompensating filter device is also an important factor for a correctexposure equalization over the entire radiographic image. In the priorart one has generally used absorption bodies of aluminium. This has thedisadvantage, however, that those portions of theradiation that passthrough thin and low-absorbing portions of the object being radiographedand that consequently pass through portions of the compensating filterdevice having a high absorption will be subject to a displacement of theenergy distribution spectrum of the radiation towards higher energyvalues, that is towards a harder radiation. As this harder radiationpenetrates the object more easily, the portions of the object having alow absorption, that is generally the thinner portions of the object,will be reproduced on the radiograph with a lower image contrast thenthe portions of the object having a higher absorption, that is generallythe thicker portions of the object. This can be avoided, however, byselecting as radiation absorbing substance in the compensating filterdevice a substance having a K-absorption edge located within the energydistribution spectrum of the radiation used for the radiographicexposure and preferably close to the energy value of the intensitymaximum of the radiation being used. For

X-ray radiation this means that the radiation absorbing substance shallhave an absorption edge corresponding to an energy, which multipliedwith a factor of 1.2 to 2.0, preferably a factor of about 1.4, gives thevoltage used on the X-ray tube during the radiographic exposure.However, this value is not critical, but the tube voltage may varywithin a comparatively wide range without the contrast improving effectbeing lost. Suitable radiation absorbing substances are the rare earthmetals, which satisfy the above conditions for tube voltages normallyused for radiography of skeleton structures and also for many softtissue structures.

What we claim is:

1. In a radiographic apparatus including a radiation source, an objectplane for an object to be radiographed and an image plane for an imagerecording medium (5), a device for equalizing the average exposure ofdifferent portions of said image recording medium, comprising acompensating filter device disposed in the radiation path between saidradiation source and said object plane and including radiation absorbingmeans having a variable form such that the absorption values of saidradiation absorbing means within different portions of the radiationbeam from said radiation source can be varied substantiallyindependently of other portions, control means (7) for varying theabsorption values of said compensating filter device within differentportions of the radiation beam by varying the form of said radiationabsorbing means in response to control signals supplied to said controlmeans, and radiation detecting means located beyond said object plane asseen from said radiation source for sensing the average intensity valuesof different portions of the radiation beam and generating outputsignals representing said average intensity values, the con trol signalsfor said control means determining the absorption values of saidcompensating filter device (6) within different portions of theradiation beam being derived from the output signals of said radiationdetector means.

2. A device as claimed in claim 1, wherein said radiation absorbingmeans of said compensating filter device include a plurality of solidbodies of radiation absorbing material mounted so as to be movablerelative to the radiation beam and each other in a plane substantiallyperpendicular to the direction of radiation, and said control meansinclude servomotor means coupled to'said absorption bodies fordetermining their positions.

3. A device as claimed in claim 1, wherein said radiation absorbingmeans of said compensating filter device include a layer of a formableradiation absorbing material disposed in a plane substantiallyperpendicular to the direction of radiation, and said control meansinclude means for varying the thickness of said layer in the directionof radiation within different sections of the layer.

4. A device as claimed in claim 3, wherein said radiation absorbingmeans include a flat chamber arranged in a plane substantiallyperpendicular to the direction of radiation and filled with a radiationabsorbing liquid, one of the major walls of said chamber consisting of aresiliently flexible diaphragm, and said control means being coupled tosaid diaphragm in a plurality of spaced points on the surface of thediaphragm for varying the distance of the diaphragm at said points fromthe opposite major wall of said chamber.

5. A device as claimed in claim 4, wherein said control means include aplurality of servomotors and associated Bowden cables, each of saidBowden cables having its one end coupled to the associated servomotorand its opposite end attached to said flexible diaphragm in one of saidpoints thereon, whereby each servomotor can through its associatedBowden cable exert alternatively a pulling or a pushing forcesubstantially parallel to the direction of radiation upon said diaphragmin the point of connection of the Bowden cable to the diaphragm.

6. A device as claimed in claim 4, wherein the points of connections ofsaid control means to said diaphragm are arranged in a triangular gridpattern array.

7. A device as claimed in claim 4, wherein said flexible diaphragm has asmaller rigidity within portions of the diaphragm located along thejunction lines between the connection points of said control means tothe diaphragm than within portions of the diaphragm enclosed by saidjunction lines.

8. A device as claimed in claim 3, wherein said radiation absorbingmeans include a flat tray mounted in a plane substantially perpendicularto the direction of radiation and a moldable layer of a radiationabsorbing material supported on said tray, and said control meansinclude means for varying the thickness of said layer by molding theupper surface thereof.

9. A device as claimed in claim 8, wherein said means for molding theupper surface of said moldable layer of radiation absorbing material onsaid tray include an elongate resiliently flexible scraper meansextending across said tray perpendicularly to the direction of radiationand movable relative the tray in a direction perpendicular to thedirection of radiation and the longitudinal direction of the scrapermeans, and said control means include a plurality of servomotors(40a-40e) coupled to said scraper means in spaced points along itslength for varying the distance between the bottom of said tray and thescraping edge of said scraper means.

10. A device as claimed in claim 9, comprising dis pensing means movableover said tray for dispensing said radiation absorbing material ontosaid tray through an elongate slot-shaped dispenser opening, saidflexible scraping means forming the upper edge of said dispenseropening.

1 1. A device as claimed in claim 9, wherein said radiation absorbingmaterial consists of an intrinsically loose and freely moving powder,said tray is provided with a foramenous bottom premeable to air butimpervious to said powder, means are provided for producing a reducedair pressure underneath said bottom, whereby an air pressure gradient isestablished across the powder layer on said tray maintaining said powderlayer in a substantially stable state, said scrapping means is adaptedwhen moving over the tray to cut down into said powder layer (36) to adepth determined by the distance between said bottom of the tray andsaid scraping means, and means are provided for removing the portion ofsaid powder layer located above the cutting edge of said scraping means.i

12. A device as claimed in claim 11, wherein said means for removingsaid portion of said powder layer include an elongate suction nozzleextending along said scraping means above the cutting edge thereof.

13. A device as claimed in claim 9, wherein said radiation detectingmeans include elongate radiation detectors extending parallel to eachother and to the direction of movement (38) of said scraping means (39),a diaphragm (43,44) provided with a slot-shaped aperture extendingparallel to the longitudinal direction of said scraping means beingmovable relative to the radiation beam in the same direction as saidscraping means and in synchronism therewith in such a manner that theportion of the radiation beam passing through said diaphragm aperture isidentical with the portion of the radiation beam passing through saidlayer of radiation absorbing material on said tray close to saidscraping means.

14. A device as claimed in claim 1, wherein said radiation absorbingmeans include a flat tray mounted in a plane substantially perpendicularto the direction of radiation for supporting a layer of an intrinsicallyloose and freely moving, radiation absorbing powder, said tray beingprovided with a foramenous bottom permeable to air but impervious tosaid powder, and means for producing a reduced air pressure underneathsaid bottom, whereby an air pressure gradient is established across saidpowder layer on said bottom maintaining said layer in a substantiallystable state, and said control means include means responsive to saidsignals from said radiation detecting means for depositing said powderupon said bottom of said tray in a layer having a thickness within eachportion of said tray determined by the radiation intensity beyond saidobject plane of the portion of the radiation beam passing through saidportion of the tray.

15. A device as claimed in claim 14, wherein said powder depositingmeans include means for producing a narrow jet of said powder directedtowards said tray and means for moving said powder jet over the bottomof the tray along a predetermined scanning pattern.

16. A device as claimed in claim 15, wherein said means (54) for movingsaid powder jet include means (62,63) for electrostatic deflection ofthe powder jet.

17. A device as claimed in claim 15, wherein said radiation detectingmeans include means for electronically scanning the radiation image ofsaid object beyond said object plane along a predetermined scanningpattern and producing an electric video signal repre-' senting theintensity value of the portion of the radiation image being scanned atany moment, said means for moving said powder jet being controlled bysaid radiation image scanning means to move the powder jet along ascanning pattern over said tray corresponding to the scanning patternfor said radiation image, and said powder jet being controlled inresponse to said video signal so as to deposit an amount of powder uponsaid tray during its scanning motion determined by the amplitude of saidvideo signal.

18. A device as claimed in claim 17, comprising means for varying theflow rate of said powder jet in response to said video signal.

19. A device as claimed in claim 1, wherein said control signalssupplied to said control means are propor' tional to differences betweenthe output signals of said radiation detecting means and a referencesignal.

20. A device as claimed in claim 19, comprising a radiation detectorlocated beyond said object plane as seen from said radiation source forgenerating said reference signal.

21. A device as claimed in claim 20, wherein said radiation detectorgenerating said reference signal is disposed to be affected by a portionof the radiation beam having an intensity which is substantiallyindependent of the varying absorption values of said compensating filterdevice.

22. A device as claimed in claim 1, wherein said control means areassociated with signal generating means responsive to the operation ofsaid control means for generating signals representing the absorptionvalues of said radiation absorbing means determined by said controlmeans, said signals being supplied as a. negative feedback to the inputof said control means.

23. A device as claimed in claim 22, wherein the feedback factor isvariable.

24. A device as claimed in claim 1 for a radiographic apparatusincluding a variable pirmary diaphragm for restricting the radiationbeam from said radiation source, comprising signal modifying meansaffected by the setting of said primary diaphragm for modifying saidcontrol signals supplied to said control means from said radiationdetecting means in a manner making the adjustment of said compensatingfilter device by said control means substantially independent of thesetting of said primary diaphragm.

25. A device as claimed in claim 1, wherein said radiation detectingmeans are located beyond said image plane as seen from said radiationsource.

26. A device as claimed in claim 1, wherein said radiation detectingmeans are disposed between said object plane and said image plane andare removable from the path of the radiation beam.

27. A device as claimed in claim 1, in a radiographic apparatusincluding an automatic exposure control system operating in response tothe output signals of said radiation detecting means.

28. A device as claimed in claim 1, wherein said radiation absorbingmeans comprises at least one element having a K absorption edge withinthe energy spectrum of the radiation used for the radiographic exposureof the object.

29. A device as claimed in claim 28, wherein the K absorption edge ofsaid radiation absorbing element is located close to the energy valuefor the intensity maximum of the radiation being used for theradiographic exposure of the object.

30. A device as claimed in claim 28, wherein the K absorption edge ofsaid radiation absorbing element corresponds to an energy whichmultiplied with a factor of 1.2 to 2.0, preferably a factor of about1.4, corresponds to the voltage of an X-ray tube used as said radiationsource for. the radiographic exposure of the object. I

31. A device as claimed in claim 28, wherein said radiation absorbingelement is a rare earth metal.

l i I!

1. In a radiographic apparatus including a radiation source, an objectplane for an object to be radiographed and an image plane for an imagerecording medium (5), a device for equalizing the average exposure ofdifferent portions of said image recording medium, comprising acompensating filter device disposed in the radiation path between saidradiation Source and said object plane and including radiation absorbingmeans having a variable form such that the absorption values of saidradiation absorbing means within different portions of the radiationbeam from said radiation source can be varied substantiallyindependently of other portions, control means (7) for varying theabsorption values of said compensating filter device within differentportions of the radiation beam by varying the form of said radiationabsorbing means in response to control signals supplied to said controlmeans, and radiation detecting means located beyond said object plane asseen from said radiation source for sensing the average intensity valuesof different portions of the radiation beam and generating outputsignals representing said average intensity values, the control signalsfor said control means determining the absorption values of saidcompensating filter device (6) within different portions of theradiation beam being derived from the output signals of said radiationdetector means.
 2. A device as claimed in claim 1, wherein saidradiation absorbing means of said compensating filter device include aplurality of solid bodies of radiation absorbing material mounted so asto be movable relative to the radiation beam and each other in a planesubstantially perpendicular to the direction of radiation, and saidcontrol means include servomotor means coupled to said absorption bodiesfor determining their positions.
 3. A device as claimed in claim 1,wherein said radiation absorbing means of said compensating filterdevice include a layer of a formable radiation absorbing materialdisposed in a plane substantially perpendicular to the direction ofradiation, and said control means include means for varying thethickness of said layer in the direction of radiation within differentsections of the layer.
 4. A device as claimed in claim 3, wherein saidradiation absorbing means include a flat chamber arranged in a planesubstantially perpendicular to the direction of radiation and filledwith a radiation absorbing liquid, one of the major walls of saidchamber consisting of a resiliently flexible diaphragm, and said controlmeans being coupled to said diaphragm in a plurality of spaced points onthe surface of the diaphragm for varying the distance of the diaphragmat said points from the opposite major wall of said chamber.
 5. A deviceas claimed in claim 4, wherein said control means include a plurality ofservomotors and associated Bowden cables, each of said Bowden cableshaving its one end coupled to the associated servomotor and its oppositeend attached to said flexible diaphragm in one of said points thereon,whereby each servomotor can through its associated Bowden cable exertalternatively a pulling or a pushing force substantially parallel to thedirection of radiation upon said diaphragm in the point of connection ofthe Bowden cable to the diaphragm.
 6. A device as claimed in claim 4,wherein the points of connections of said control means to saiddiaphragm are arranged in a triangular grid pattern array.
 7. A deviceas claimed in claim 4, wherein said flexible diaphragm has a smallerrigidity within portions of the diaphragm located along the junctionlines between the connection points of said control means to thediaphragm than within portions of the diaphragm enclosed by saidjunction lines.
 8. A device as claimed in claim 3, wherein saidradiation absorbing means include a flat tray mounted in a planesubstantially perpendicular to the direction of radiation and a moldablelayer of a radiation absorbing material supported on said tray, and saidcontrol means include means for varying the thickness of said layer bymolding the upper surface thereof.
 9. A device as claimed in claim 8,wherein said means for molding the upper surface of said moldable layerof radiation absorbing material on said tray include an elongateresiliently flexible scraper means extending across said trayperpendicularly to the Direction of radiation and movable relative thetray in a direction perpendicular to the direction of radiation and thelongitudinal direction of the scraper means, and said control meansinclude a plurality of servomotors (40a-40e) coupled to said scrapermeans in spaced points along its length for varying the distance betweenthe bottom of said tray and the scraping edge of said scraper means. 10.A device as claimed in claim 9, comprising dispensing means movable oversaid tray for dispensing said radiation absorbing material onto saidtray through an elongate slot-shaped dispenser opening, said flexiblescraping means forming the upper edge of said dispenser opening.
 11. Adevice as claimed in claim 9, wherein said radiation absorbing materialconsists of an intrinsically loose and freely moving powder, said trayis provided with a foramenous bottom premeable to air but impervious tosaid powder, means are provided for producing a reduced air pressureunderneath said bottom, whereby an air pressure gradient is establishedacross the powder layer on said tray maintaining said powder layer in asubstantially stable state, said scrapping means is adapted when movingover the tray to cut down into said powder layer (36) to a depthdetermined by the distance between said bottom of the tray and saidscraping means, and means are provided for removing the portion of saidpowder layer located above the cutting edge of said scraping means. 12.A device as claimed in claim 11, wherein said means for removing saidportion of said powder layer include an elongate suction nozzleextending along said scraping means above the cutting edge thereof. 13.A device as claimed in claim 9, wherein said radiation detecting meansinclude elongate radiation detectors extending parallel to each otherand to the direction of movement (38) of said scraping means (39), adiaphragm (43,44) provided with a slot-shaped aperture extendingparallel to the longitudinal direction of said scraping means beingmovable relative to the radiation beam in the same direction as saidscraping means and in synchronism therewith in such a manner that theportion of the radiation beam passing through said diaphragm aperture isidentical with the portion of the radiation beam passing through saidlayer of radiation absorbing material on said tray close to saidscraping means.
 14. A device as claimed in claim 1, wherein saidradiation absorbing means include a flat tray mounted in a planesubstantially perpendicular to the direction of radiation for supportinga layer of an intrinsically loose and freely moving, radiation absorbingpowder, said tray being provided with a foramenous bottom permeable toair but impervious to said powder, and means for producing a reduced airpressure underneath said bottom, whereby an air pressure gradient isestablished across said powder layer on said bottom maintaining saidlayer in a substantially stable state, and said control means includemeans responsive to said signals from said radiation detecting means fordepositing said powder upon said bottom of said tray in a layer having athickness within each portion of said tray determined by the radiationintensity beyond said object plane of the portion of the radiation beampassing through said portion of the tray.
 15. A device as claimed inclaim 14, wherein said powder depositing means include means forproducing a narrow jet of said powder directed towards said tray andmeans for moving said powder jet over the bottom of the tray along apredetermined scanning pattern.
 16. A device as claimed in claim 15,wherein said means (54) for moving said powder jet include means (62,63)for electrostatic deflection of the powder jet.
 17. A device as claimedin claim 15, wherein said radiation detecting means include means forelectronically scanning the radiation image of said object beyond saidobject plane along a predetermined scanning pattern and producing anelectric video signal representing the intensity value of the portion ofthe radiation image being scanned at any moment, said means for movingsaid powder jet being controlled by said radiation image scanning meansto move the powder jet along a scanning pattern over said traycorresponding to the scanning pattern for said radiation image, and saidpowder jet being controlled in response to said video signal so as todeposit an amount of powder upon said tray during its scanning motiondetermined by the amplitude of said video signal.
 18. A device asclaimed in claim 17, comprising means for varying the flow rate of saidpowder jet in response to said video signal.
 19. A device as claimed inclaim 1, wherein said control signals supplied to said control means areproportional to differences between the output signals of said radiationdetecting means and a reference signal.
 20. A device as claimed in claim19, comprising a radiation detector located beyond said object plane asseen from said radiation source for generating said reference signal.21. A device as claimed in claim 20, wherein said radiation detectorgenerating said reference signal is disposed to be affected by a portionof the radiation beam having an intensity which is substantiallyindependent of the varying absorption values of said compensating filterdevice.
 22. A device as claimed in claim 1, wherein said control meansare associated with signal generating means responsive to the operationof said control means for generating signals representing the absorptionvalues of said radiation absorbing means determined by said controlmeans, said signals being supplied as a negative feedback to the inputof said control means.
 23. A device as claimed in claim 22, wherein thefeedback factor is variable.
 24. A device as claimed in claim 1 for aradiographic apparatus including a variable pirmary diaphragm forrestricting the radiation beam from said radiation source, comprisingsignal modifying means affected by the setting of said primary diaphragmfor modifying said control signals supplied to said control means fromsaid radiation detecting means in a manner making the adjustment of saidcompensating filter device by said control means substantiallyindependent of the setting of said primary diaphragm.
 25. A device asclaimed in claim 1, wherein said radiation detecting means are locatedbeyond said image plane as seen from said radiation source.
 26. A deviceas claimed in claim 1, wherein said radiation detecting means aredisposed between said object plane and said image plane and areremovable from the path of the radiation beam.
 27. A device as claimedin claim 1, in a radiographic apparatus including an automatic exposurecontrol system operating in response to the output signals of saidradiation detecting means.
 28. A device as claimed in claim 1, whereinsaid radiation absorbing means comprises at least one element having a Kabsorption edge within the energy spectrum of the radiation used for theradiographic exposure of the object.
 29. A device as claimed in claim28, wherein the K absorption edge of said radiation absorbing element islocated close to the energy value for the intensity maximum of theradiation being used for the radiographic exposure of the object.
 30. Adevice as claimed in claim 28, wherein the K absorption edge of saidradiation absorbing element corresponds to an energy which multipliedwith a factor of 1.2 to 2.0, preferably a factor of about 1.4,corresponds to the voltage of an X-ray tube used as said radiationsource for the radiographic exposure of the object.
 31. A device asclaimed in claim 28, wherein said radiation absorbing element is a rareearth metal.